An atom probe, also referred to as an atom probe microscope, is a device which allows specimens to be analyzed on an atomic level. A typical atom probe includes a specimen mount, a local electrode, and a detector. During typical analysis, a specimen is situated in the specimen mount and a positive electrical charge (e.g., a baseline voltage) is applied to the specimen such that the electrostatic field near the apex of the specimen is approximately 90% of that required to spontaneously ionize surface atoms (generally on the order of 5 to 50 volts per nanometer). The detector is spaced apart from the specimen and is either grounded or negatively charged. The local electrode is located between the specimen and the detector, and is either grounded or negatively charged. (The local electrode is sometimes referred to as a “counter electrode” or “extraction electrode”; additionally, because electrodes in an atom probe typically serve as electrostatic lenses, the term “lens” is sometimes used in place of the term “electrode.”) A positive electrical pulse (above the baseline voltage), a laser pulse (e.g., photonic energy), and/or another pulsed form of ionization energy (e.g., an electron beam or packet, ion beam, RF pulse, etc.) is intermittently applied to the specimen to increase the probability that surface atoms on the specimen will ionize. Alternatively or additionally, a negative voltage pulse can be applied to the local electrode in synchrony with the foregoing energy pulse(s). Occasionally, a pulse will cause ionization of a single atom near the tip of the specimen. The ionized atom(s) separate or “evaporate” from the surface, pass through an aperture in the local electrode, and impact the surface of the detector, typically a microchannel plate (MCP). The elemental identity of an ionized atom can be determined by measuring its time of flight (TOF), the time between the pulse that liberates the ion from the surface of the specimen and the time it impinges on the detector. The velocity of the ions (and thus their TOF) varies based on the mass-to-charge-state ratio (m/n) of the ionized atom, with lighter and/or more highly charged ions taking less time to reach the detector. Since the TOF of an ion is indicative of the mass-to-charge ratio of the ion, which is in turn indicative of elemental identity, the TOF can help identify the composition of the ionized atom. In addition, the location of the ionized atom on the surface of the specimen can be determined by measuring the location of the atom's impact on the detector. Thus, as the specimen is evaporated, a three-dimensional map or image of the specimen's constituent atoms can be constructed. While the image represented by the map is a point projection, with atomic resolution and a magnification of over 1 million times, the map/image data can be analyzed in virtually any orientation, and thus the image can be considered more tomographic in nature. Further details on atom probes can be found, for example, in U.S. Pat. Nos. 5,440,124; 7,157,702; 7,683,318; 7,884,323; 8,074,292; 8,276,210; 8,513,597; and 8,575,544, as well as in the patents and other literature referenced therein.
One of the most important specifications of an atom probe is its mass resolving power, i.e., the ability to discern one ionic species from another. In general, a mass resolving power of 500 or better is desired for most applications, where the mass resolving power is defined as m/Δm at full-width-half-maximum. Another important specification is its field of view, i.e., the area of the specimen imaged by the detector (or, stated differently, the area of the specimen from which ions can be collected with reasonable correlation to their original positions on the specimen). Field of view can be increased by decreasing the length of the ion flight path between the specimen and the detector, but this comes at a cost to mass resolving power, which benefits from longer flight paths (and thus longer TOF). TOF can be increased with the use of local electrodes, pulsed lasers, or other sources of ionization energy that allow the application of lower voltages to the specimen (and thereby decrease the departure speed of ions evaporating from the specimen), thereby allowing closer spacing of the specimen and detector (and greater field of view) with lesser degradation of mass resolving power. However, as of late 2014, high-performance atom probes using local electrodes and lasers typically have an angular field of view of no greater than 40-60 degrees full-angle, with maximum mass resolving power of 700-800 at the center of the field of view (and decreasing away from the center). Mass resolution can also be enhanced by use of energy compensating electrodes/lenses, such as a Poschenrieder lens (e.g., U.S. Pat. No. 3,863,068) or a reflectron (e.g., U.S. Pat. No. 6,740,872). These lenses bend or reflect the flight path and allow longer TOF, but typically have a very narrow field of view due to the limited acceptance angle of these lenses (the acceptance angle being the angle defined by the outer bounds of the cone of ions emitted by the specimen). The reflectron of U.S. Pat. No. 8,134,119 has a unique curved surface which provides a large acceptance angle, allowing a field of view of approximately 50 degrees full-angle, while simultaneously providing a mass resolving power of 1000 or more.
However, in order to collect all (or nearly all) ions emitted from a specimen apex, a field of view of approximately 100 degrees (full angle) is needed. Such a “full field of view” atom probe was not known to exist, and the means for constructing such an atom probe—even with vastly inferior mass resolving power—was unknown.